In the art of fuel metering, it is known to measure the flow rate of induction air by means of a venturi section which provides a subambient pressure and to employ the subambient pressure for inducing flow of fuel across an orifice such that the flow of fuel is proportional to the flow of air. In practical systems for use with internal combustion engines, it is sometimes necessary to restrict the venturi throat more than would be desirable for proper breathing of the engine in order to provide a pressure signal of sufficient strength for inducing proper fuel flow in the typical operating range of the engine. Further it is generally accepted practice to provide an idle system for metering fuel at low speeds since at low speeds the flow rate of inducted air passing through the venturi section is not sufficient to provide an accurate metering signal. The venturi signal varies in the sense of being stronger at higher flow rates and weaker at lower flow rates.
It is also known to employ a subambient pressure signal derived from the intake manifold of an engine in combination with a fuel injector for metering fuel. The manifold pressure signal varies in the sense of being stronger at lower flow rates and weaker at higher flow rates.
Venturi pressure signals are typically employed with carburetor systems while manifold pressure signals are typically employed with injection systems. Thus, although venturi and manifold pressure signals vary in opposite sense, ordinarily they are not employed to complement each other because they are used in different fuel metering systems.
The prior art includes a fuel system as disclosed in U.S. Pat. No. 3,687,121 wherein fluid logic means are connected for receiving a venturi pressure signal which can be subjected to amplification and modification for use with an injection system.